Developing processes of electrophotography are divided into a so-called one-component developing process using primarily a toner, and a so-called two-component developing process using a mixture of a toner with a glass bead, a magnetic carrier, or a coat carrier made of the glass bead or the magnetic carrier wherein their surface is coated with resin or the like.
In such a two-component developing process, a carrier is used, and thus a two-component developer has a wider area frictionally charged to toner. In addition, the two-component developing process is more stable in charge property than the one-component developing process and is advantageous in providing high-quality images over a long period of time and has a high-ability of supplying a toner to areas to be developed. Thus, the two-component developing process is frequently used particularly in high-speed machine.
In an electrophotographic system employing a so-called digital method wherein a latent electrostatic image is formed on an image bearing member using a laser beam or the like and the latent electrostatic image is visualized, the two-component developing method utilizing the above-noted features is also widely employed.
In recent years, demands of higher stabilization and better quality of electrophotographic images have been increased. Especially, a developments of image developing system capable of high-fidelity latent image development has become extremely important in minimizing the minimal unit (one dot) of latent images and increasing the density in order to enhancements the quality of images. Furthermore, reduction in the dispersion of electrification distribution has became important on stabilization of image quality.
There have been various proposals on the use of a small diameter carrier, as minimizing the particle diameter of carrier is considered as an effective way for high-fidelity latent image development.
For example, Patent Literature 1 proposes a magnetic carrier made of ferrite particles with spinel structures and an average particle diameter of less than 30 μm, however, the proposed carrier is not coated with resin and is used under low-electric field applied thereon, and has disadvantages in that it has poor developing ability and, because its is not coated with resin, it has a short operating life.
Furthermore, Patent Literature 2 proposes an electrophotographic carrier having carrier particles having the 50% average particle diameter (D50) in the range of 15 μm to 45 μm, the carrier contains carrier particles having a particle diameter less than 22 μm in the range of 1% to 20%, less than 16 μm in the range of 3% or less, 62 μm or more in the range of 2% to 15%, 88 μm or more in the range of 2% or less, and the specific surface area S1 of the carrier determined by air permeability method and the specific surface area S2 of the carrier calculated by the equation S2=(6/ρ·D50)×104 (ρ represents a specific gravity of carrier) satisfy the condition 1.2≦S1/S2≦2.0.
When the above-noted carrier with small particle diameters is used, there are the following advantages:
(1) Sufficient frictional charges can be given to individual toner particles because the carrier has a large surface area per unit volume, and the carrier has fewer occurrences of being low-charged and/or oppositely-charged. As a result, fewer background smears occur, and because of less quantity of toner dusts in the areas around dots and image blurs, the carrier can provide excellent dot reproductivity;
(2) The average charge amount of toner can be lowered because the carrier has a large surface area per unit volume and less occurrence of background smear, providing sufficient image densities; and
(3) A dense magnetic brush can be formed because of the carrier having a small particle diameter. And the excellent flowability of the magnetic brush will reduce the occurrence of magnetic brush trails left on an image surface.
However, the related proposed carrier having small particle diameters has disadvantages in that carrier adhesion easily occurs, causing occurrences of image bearing member flaws and fixing roller flaws, thus implementation of the related proposed carriers is difficult.
In particular, when a carrier having an average particle diameter of less than 32 μm is used, the carrier surface texture will be drastically improved, and a high image quality can be obtained, however, there is a problem wherein carrier adhesion occurs very easily.    (Patent Literature 1;) Japanese Patent Application Laid-Open No. 58-144839    (Patent Literature 2;) Japanese Patent No. 3029180